In U.S. Pat. Nos. 6,802,820 and 6,802,971 and application Ser. No. 10/959,918, now U.S. Pat. No. 7,195,711, there are described hollow fiber membranes capable of in-vivo plasmapheresis and/or in-vivo ultrafiltration. The aforesaid patents and application are incorporated herein by reference in their entirety, respectively. The described membranes comprise elongated hollow microporous fibers having an asymmetrical pore size and asymmetrical mass density between the inner and outer wall surfaces with a higher mass density adjacent to the outer wall and a lower mass density adjacent to the inner wall. The wall structure is also described as a continuous change in mass density between the outer fiber surface and the inner lumen surface with the pore size gradually changing between these inner and outer wall surfaces. The fibers are capable of separating blood plasma and toxins or metabolic waste from whole blood within a blood vessel by passing the plasma through the fiber wall from the outer wall surface to the interior lumen. The aforesaid asymmetrical hollow fiber membranes are used in filter devices to be implanted in the vasculature of a patient, such filter devices described in U.S. Pat. No. 6,899,692, incorporated by reference herein in its entirety. The use of such membranes and filter devices in apparatus and systems for therapeutic apheresis and patient fluid management are described in U.S. Pat. No. 6,849,183 and copending application Ser. No. 11/078,016, filed Mar. 11, 2005, now U.S. Pat. No. 7,481,936, both of which are incorporated by reference herein in their entirety, respectively.
During use, the relatively small pores at or adjacent to the outer fiber surface become clogged or occluded with particulate matter from the permeate (blood) whereby performance of the membranes is generally degraded over time. At least some of such clogging or fouling of the filter membranes is a result of blood clot formation (thrombosis) and platelet aggregation at the fiber surface. U.S. Pat. No. 6,659,973 is directed to apparatus and methods for periodically backflushing the hollow fibers to cleanse the membrane pores by removing the occlusions and restoring fiber performance. The description of the aforesaid '973 patent is incorporated by reference herein in its entirety. The backflush fluid may also be supplied with an anticoagulant such as heparin to assist in reducing blood clot formation and platelet aggregation at the fiber surface as described in copending application Ser. No. 10/408,657 filed Apr. 4, 2003, published as U.S. Patent Publication No. US-2003-0236482-A1 and now abandoned, also incorporated herein by reference. However, because the use of heparin in the backflush may increase the risk of bleeding, it may not be suitable for some patients.